Polyisocyanates of the diphenylmethane series are generally understood to include isocyanates and mixtures of isocyanates of the following type:
Similarly, polyamines of the diphenylmethane series are generally understood to include compounds and mixtures of compounds of the following type:
The industrial production of isocyanates by the reaction of amines with phosgene in solvents is known and is described in detail in the literature (Ullnanns Enzyklopädie der technischen Chemie, 4th edition, volume 13, pages 347–357, Verlag Chemie GmbH, Weinheim, 1977). Based on this process, a mixture of polyisocyanates is produced which is used as the polyisocyanate component in the production of polyurethane foams and other polyurethane plastics produced by polyaddition processes.
The continuous, non-continuous or semi-continuous production of polyamines of the diphenylmethane series, also referred to below as MDA, is described in numerous patents and publications. The production conventionally takes place by reacting aniline and formaldehyde in the presence of acidic catalysts. HCl is conventionally used as the acidic catalyst. According to the prior art, the acidic catalyst is neutralised at the end of the process before the final reprocessing steps (such as e.g. the removal of excess aniline by distillation) by adding a base, and is thus consumed.
The main products of the acid-catalysed reaction of aniline and formaldehyde are the diamine 4,4′-MDA, its positional isomers 2,4′-MDA and 2,2′-MDA and higher homologues of polyamines of the diphenylmethane series. The polyisocyanates of the diphenylmethane series, referred to below as MDI, are produced by phosgenation of the corresponding polyamines. The polyisocyanates of the diphenylmethane series produced in this way contain the various isocyanate isomers and their higher homologues in the same composition as the polyamines from which they were produced. The controlling variable for influencing the isomer distribution is the quantity of acidic catalyst used in the process during the acid-catalysed reaction of aniline and formaldehyde. To be able to produce MDI with the desired isomer distribution, therefore, it is sometimes necessary to use considerable quantities of acidic catalyst and correspondingly considerable quantities of base for neutralising the acidic catalyst. This further leads to quite large quantities of salt-containing waste water streams and, correspondingly, high costs for reprocessing and disposal.
For some time, it has been the goal of numerous trials and studies described in the literature to find processes to avoid or alleviate this problem. Thus, for example, WO-A1-0174755 describes the production of polyamines of the diphenylmethane series in the presence of heterogeneous catalysts, which take on the role of the acidic catalyst. In contrast to the homogeneous catalysts conventionally used, this type of catalyst can be separated from the reaction mixture by simple means and does not then have to be neutralised before being reprocessed. However, a disadvantage of this process is that the acidic solid deactivates over time and that the range of products obtainable using these catalysts is limited. EP-A1-1167343 describes MDA production according to the prior art, extended by an additional separation of the 2,4′-MDA and 2,2′-MDA isomers, and also their reaction with formaldehyde and recycling of this mixture to the beginning of the process. The recycled isomers are thus preferentially converted to higher-molecular-weight MDA components. This means that the close association between catalyst use and formation of 2,4′- and 2,2′-MDA is moderated by introducing another controlling variable. A disadvantage of this process, however, is that the conversion of the recycled isomers to higher MDA homologues can have a negative effect on the product properties of the MDI produced by subsequent phosgenation, and that additional outlay on apparatus is needed for distillation for the separation of isomers.
The object of the present invention was therefore to provide a process for the production of polyisocyanates of the diphenylmethane series, in which the consumption of acidic catalyst and correspondingly of base for neutralising the acidic catalyst at the MDA stage can be reduced or avoided.